V.sub.be, the base to emitter voltage of a PN junction, is complementary to absolute temperature (CTAT) ##EQU1## where K is a constant, T is absolute temperature, q is charge, I is the collector current and I.sub.s is the saturation current. .DELTA.V.sub.be, the difference between the V.sub.be 's of two PN junctions, is proportional to absolute temperature (PTAT) ##EQU2##
It is this relationship that underlies the use of PTAT cells to measure temperature. In one approach the cell uses two PN junctions of different areas ratioed by N and supplied with the same current to develop .DELTA.V.sub.be as shown by Paul Brokaw, "A Simple Three-Terminal IC Bandgap Reference", Solid-State Circuits, Vol. SC-9, pp. 388-393 (December 1974). This works well but the ratio of the areas must be carefully controlled to ensure accuracy. Increased .DELTA.V.sub.be can be obtained by increasing the ratio but doubling the .DELTA.V.sub.be requires squaring the ratio of the areas which increases the size of the cell and makes the precision of the ratio difficult to maintain. In another approach this problem with the ratioed areas is avoided by using but a single PN junction supplied with two different currents in a preselected ratio to obtain .DELTA.V.sub.be. An improvement on these approaches disclosed in U.S. Pat. No. 5,982,221; Switched Current Temperature Sensor Circuit with Compounded .DELTA.V.sub.be, of Michael G. Tuthill, filed uses two different currents and two identical PN junctions to obtain an improved, larger, 2 .DELTA.V.sub.be signal and further teaches alternately switching the .DELTA.V.sub.be inputs to a differential amplifier to accommodate ever larger signals of 4 .DELTA.V.sub.be, 6 .DELTA.V.sub.be, 8 .DELTA.AV.sub.be, and so on. All of these dual junction approaches suffer from the problem that the two PN junctions must be on the same chip as they produce a temperature value which is the average of the temperature of the two junctions. That is, they cannot be properly used to measure off-chip temperature.